Sensory feedback regarding limb position is critical for normal locomotion and adaptation to external perturbations during movement. Feedback relating to rapid changes in muscle stretch, conveyed to the spinal cord by proprioceptive Ia sensory afferents, is especially important in this regard. While Ia afferents make some monosynaptic connections with motor neurons (MNs), direct proprioceptive influence on MN activity is exceptional; in general, proprioceptive feedback to MNs is indirect via interneuronal circuits of the spinal cord. These circuits integrate sensory feedback with centrally generated patterns of activity to modulate MN responses. Despite the importance of these more complex circuits in shaping motor output, very little is known about how these circuits are formed. Developmental mechanisms that control the establishment of proprioceptive input to interneuronal circuits, even those containing a single interneuron class, are virtually unknown. One simple sensory-motor circuit is responsible for reciprocal inhibition (RI), which acts to prevent co- contraction of antagonist flexor and extensor muscles at a joint. This sensory-motor circuit contains a single class of interneuron, the glycinergic Ia inhibitory interneuron (IaIN). These interneurons receive sensory information about muscle stretch through monosynaptic inputs from Ia afferents supplying specific muscles. Normal development of RI requires assembly of multiple, modular circuits responding to activation of Ia afferents arising from specific muscles and inhibiting functionally appropriate MN targets. What developmental mechanisms guide this process? Activity-induced RI circuit modulation in early postnatal animals suggests the possibility that activity-dependent mechanisms may be involved in the establishment of Ia sensory afferent connections with functionally appropriate subsets of IaINs. Experiments presented in this proposal will directly test this hypothesis by investigating the status of afferent connectivity with RI circuits at the initial stages of its development. We will test the necessity and sufficiency of proprioceptive afferent activity in the process of afferent segregation onto subsets of IaINs using genetic strategies in mice. Development of reciprocal inhibition mediated by IaINs is fundamental to the development of normal locomotion, and reduced RI is associated with abnormal voluntary movement and locomotion. Understanding the progression and mechanisms of normal RI development will provide an important context and perspective for understanding the expression of abnormal motor behaviors in disease and injury states.